1. Field of the Invention
The present invention relates to a semiconductor device.
In this specification, a “semiconductor device” refers to a device that can function by utilizing semiconductor characteristics; an electro-optical device, a semiconductor circuit, and an electrical device are all included in the category of the semiconductor device.
2. Description of the Related Art
Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as thin film transistor (TFT)). The transistor is used for a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film which can be used for a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor including an amorphous oxide semiconductor film containing indium (In), gallium (Ga), and zinc (Zn) is disclosed in Patent Document 1.
Techniques for improving carrier mobility by stacking oxide semiconductor films are disclosed in Patent Documents 2 and 3.
A transistor including an oxide semiconductor film is known to have extremely low leakage current in an off state. For example, a CPU with low-power consumption utilizing the low leakage current of the transistor including an oxide semiconductor film is disclosed (see Patent Document 4).